The present invention relates to a brake motor subassembly, more particulary for a disc-brake, of the type comprising a control piston, typically intended to be sealingly slideably mounted in a bore of a brake body having an internal cavity delimited by a base wall and, at the periphery, by a stepped bore and having a mechanical actuating device acting on the control piston via an automatic compensation device formed by a screw-nut device typically intended to be placed in engagement on a threaded spindle mounted in the brake body, and stressed towards the base wall, via a washer and an axial thrust ball bearing, by a resilient member resting on the control piston at the opposite end, axially, to the base wall, and a stepped pilot piston intended to permit an inhibition of the automatic compensation in the case of excessive actuating pressure on the control piston in order to avoid an overadjustment of the brake motor, the stepped pilot piston having a first outer cylindrical peripheral surface of larger diameter sealingly slideably cooperating with an end of reduced diameter of the stepped bore and, in the vicinity of its end axially opposite to the first peripheral surface and cooperating, in contact, with the washer, a second cylindrical peripheral surface of reduced diameter cooperating sealingly slideably with an annular seal associated with a retaining washer placed resting axially against a shoulder extending radially towards the interior between the end portion of smaller diameter and the intermediate portion of the stepped bore in which the annular seal is mounted.
A brake motor subassembly of this type is described in U.S. Pat. No. 4,744,286. In the subassembly described in this document, when the stepped pilot piston is used to push back the washer counter to the resilient member and in this way interrupt the driving link between the control piston and the automatic compensation nut in the case of excessive braking pressure, the pilot piston is displaced under the effect of this braking pressure in the opposite direction to the base wall of the internal cavity of the control piston as far as a position in which a radial shoulder of the pilot piston comes to bear on a ring cooperating with an annular seal, this interrupting the displacement of the pilot piston and avoiding the crushing of the resilient member normally stressing the automatic compensation nut.
Although such a subassembly functions in an entirely satisfactory manner, its functioning characteristics nevertheless exhibit spread in series production. This spread is caused by the tolerances permitted for the manufacture of the different mechanical components and of the annular seals, this resulting in different frictions at the seals when the subassembly is subjected to the pressure of a hydraulic fluid, from which it follows finally that the neutralization of the automatic compensation occurs in a very wide range of values of the pressure of the hydraulic fluid.